If you are working with a MacBook Pro and cannot use an NVIDIA card, you won't have GPU acceleration in the Mercury Playback Engine to assist with AVCHD playback. You might try lowering the screen resolution to 1/2 or less. It works rather like Unlimited RT in FCP that way.

For field editing AVCHD with a laptop, I actually recommend a PC with a qualified NVIDIA card and 8GB of RAM or more.